Method and device for sizing a crack in a workpiece using the ultrasonic pulse-echo technique

ABSTRACT

The invention relates to a method for determining the size of a fracture ( 26 ) in a workpiece ( 20 ), in particular the depth of a fracture ( 26 ) in said workpiece ( 20 ), by means of the ultrasound pulse method, comprising the following method steps: a workpiece ( 20 ), with a front surface ( 22 ) and a back surface ( 24 ), having a fracture ( 26 ), extending from the back surface ( 24 ) and an angle test head ( 28 ) is applied to the front face ( 22 ), transmits ultrasound pulses at an angle alpha into the workpiece ( 20 ) and receives echoes from said pulse, the angle test head ( 28 ) is moved at least once over the fracture ( 26 ), such that the radiation beam ( 46 ) from the angle test head ( 28 ) completely covers the fracture ( 26 ), the received echo signals are digitised and stored in a memory ( 40 ) as variable pairs of echo signal versus runtime, the variable pairs form a value upwardly defined by an envelope ( 48 ) and the dimension of the fracture ( 26 ) is determined from the width of the envelope ( 48 ) at a given partial amplitude and the maximum amplitude of the envelope ( 48 ).

The invention relates to a method for sizing a crack in a workpieceusing the ultrasonic pulse-echo method and to a device for carrying outthis method.

The ultrasonic pulse-echo method is well known, the reader is referredto the DE-Book Krautkrämer and Krautkrämer “Werkstoffprüfung mitUltraschall” (“Material Inspection with Ultrasounds”). A probe emitsultrasonic pulses. These pulses are at least partially reflected from adiscontinuity, such as from an inner break, a crack or any othermaterial flaw, and are again received by the same probe. They areevaluated with regard to the echo amplitude, at need taking their traveltime into consideration.

It is also known to size a discontinuity using the so-calledhalf-amplitude technique. The central beam of the probe is assumed tomeet an edge of the discontinuity at the very moment when the amplitudeof the echo has dropped from the maximum value it had upon fullydetecting the discontinuity to half said value, meaning to −6 dB. Thishalf-amplitude technique however requires the movement of the proberelative to the workpiece to be registered. Additionally, the traveltime of the echo signals received may be taken into consideration forsizing a discontinuity.

A method of sizing cracks is known from the specifications of theAmerican Petroleum Institute “Recommended Practice for UltrasonicEvaluation of Pipe Imperfections”, by which an angle beam probe isdisplaced across the crack so that its radiation beam moves across thecrack. In a first way of performing this proposed measurement, in whatis termed an A-scan, the maximum echo amplitude is looked for andrecorded and the associated travel time noted down on the one side andthe travel times of the echo signals that correspond to exactly half themaximum amplitude are noted down on the other side. The method is quitecomplicated. In a second embodiment, an envelope curve is drawn, theprerequisite being an ultrasonic apparatus that has a device for storingthe maximum amplitudes such as a storage oscillograph. The thus obtainedenvelope curve is evaluated using a gate or an evaluation screen. Theevaluation screen is set to 50% of the maximum amplitude of the envelopecurve that commences at the intersection with the rising flank of theenvelope curve and ends at the intersection with the falling flank ofsaid envelope curve. To calculate the size of the flaw, the value of thesound velocity is varied until the evaluation screen sufficientlycoincides with the envelope curve. Generally, this method is describedto be lengthy and is only recommended for cracks the dimension, morespecifically the depth, of which cannot be determined otherwise. Thesize of the flaw is calculated from a formula that takes intoconsideration the product of the maximum amplitude and of the timeinterval between the two 50%-amplitudes.

Although the hereto before known methods provide dimensions for cracks,they present disadvantages in practice. This is where the inventioncomes in. It is its object to indicate a method for detecting the sizeof cracks, more specifically for detecting the depth of a crack, thatdirectly yields a value without major computation, that is, that quicklydetermines an initial value and is suited for an automatic method.

The solution to this object is achieved by

In accordance with the invention, the echo signals are digitalized andare stored in a memory as pairs of values over the travel time. If theangle beam probe sweeps across the entire crack once, one obtains aplurality of pairs of values that are limited toward the top by anenvelope curve.

In a preferred manner of performing the invention, only the maximumamplitude values for the discrete travel times are stored, that is, butthe envelope curve is stored.

The size of the flaw may now be determined directly from the envelopecurve; this can be achieved by means of a computer module provided inthe ultrasonic apparatus. The size of the flaw is proportional to theproduct of maximum amplitude and the half-width of the envelope. Theproportionality factor is determined by measuring cracks the depth ofwhich is known. This permits to find out the size of a crack in aworkpiece without major manual adjustments and irrespective of the skillof the respective ultrasonic operator. The method is suited forextensive, preferably for full, automation.

As contrasted with the prior art method, it is no longer necessary toadjust the maximum reference amplitudes to 80% of the monitor height.The maximum echo amplitudes can be measured and stored at highresolution. A half-line can be calculated directly and be displayed on amonitor. The half-width of the envelope can be directly determinedautomatically and also be displayed on the monitor.

It is possible to change the amplification factor of the ultrasonicapparatus without the computation performed in the apparatus yieldingerroneous results with regard to the crack depth. If, as this may happenin practice, the maximum echo amplitude of the flaw echo is either toohigh, that is, if it is in excess of 100%, or if it is too small (if thedynamic curve of the echo is too flat), the amplification of theultrasonic apparatus is varied. The change in the amplification dV isregistered. As the amplification changes dV, the maximum amplitude ofthe flaw echo must be converted according to formula 1A _(max) =A′ _(max)×10^(−dV/20)wherein A_(max) is the amplitude before changing the amplification andA′_(max) the amplitude after the amplitude has been changed.

It has been found out that angle beam probes having a flat emissionangle are advantageous. They need to be displaced over a larger distancethan angle beam probes having smaller emission angles.

It has been found advantageous to check the envelope obtained and storedas such in the memory by means of an evaluation device such as anevaluation screen in order to see whether the angle beam probe has beenmoved sufficiently away from the crack so as to obtain on either side ofthe envelope a value that corresponds to the echo signal without thedetected crack exerting any influence. The corresponding electronicevaluation circuit detects on the one side that the envelope falls tozero and on the other side that this actually occurs on either flank ofthe envelope. These checks can be performed automatically by the veryinspection instrument without the operator influencing them. Theinspection instrument stores the zero line obtained when no crack couldbe found. If a crack has been found, it checks whether the envelopedrops to zero on either side. If this is not the case, a correspondingsignal is delivered to the operator who reacts by moving the angle beamprobe further away from the crack that has been detected until itreaches a region in which the detected crack is no longer noticeable onthe level of the echo signal.

In an altered embodiment of the invention, an array consisting of aquite large number of individual probes is utilized instead of one anglebeam probe. Said individual probes are triggered in such a manner thateither the same effect is obtained as by displacing an angle beam probeacross the surface, that is, by having the main beam parallely offset,or that the angle of the beam is varied. In both cases, mechanicalmovement relative to the surface is no longer necessary. Put anotherway, arranging a plurality of individual probes behind each other in anarray replaces the need for displacement using one single probe.

The invention will be understood better upon reading the followingdescription with reference to exemplary embodiments. These exemplaryembodiments are not intended to limit the scope of the invention in anymanner. In the drawing:

FIG. 1 illustrates an array for ultrasonic inspection having a probethat is placed onto a workpiece having a crack; it also shows the pathof the main beam of said probe,

FIG. 2 is an illustration like FIG. 1, but in another position of theprobe relative to the crack,

FIG. 3 is an illustration of an envelope curve where the maximum echoamplitude A achieved (in Volt or %) is plotted down the side of thediagram whereas the respective travel time t (in ms) is plotted on thehorizontal axis, with detection of the half-width and of the maximumamplitude being plotted.

A workpiece 20 can be seen in the FIGS. 1 and 2, said workpiece having afront face 22 and a back face 24. Typical examples of such typeworkpieces are tubes, such as tubes with quite large diameters, e.g.,with diameters ranging from 20 to 80 cm. Typically, these tubes areoilfield pipes, tubes for pipelines, but also sheet metal and items forany application.

The crack 26 in the workpiece 22 commences on the back face 24 thereof.It also may be an internal crack that has no connection with the backface 24.

An angle beam probe 28 is placed on the front face 22. Along a main beam30 located in the centre of a radiation beam of the probe 28, it sendsultrasonic pulses into the volume of the workpiece 20 at an angle alpha.The probe 28 is what is termed a transmit-receive probe, also referredto as a T/R-probe, so that it serves both for emitting and for receivingultrasonic pulses.

The angle beam probe 28 is connected on the one side to a transmittermodule 32, also referred to as TX, and on the other side to a receivermodule 34, also referred to as RX. An analogue-to-digital converter 36,also referred to as A-D converter, is connected at the output of thereceiver module. A monitor 38 for displaying an A-scan and an envelope,which is also referred to as MON, is connected to the output of saidconverter on the one side. On the other side, there is also connected tothis output a memory 40, also referred to as MEM. Via well knownsuitable circuitry, data stored in the memory 40 can be displayed on themonitor 38, said monitor 38 however also displaying the A-scanrespectively obtained from the inspection being performed. Finally,there is provided a computer module 42 that is also referred to asmicrocomputer or μC. It is connected to all of the electronic modules ofthe ultrasonic inspection apparatus; this is shown in dashed lines. Theparts 32-42 thereby form the ultrasonic inspection apparatus. Itsstructure is actually known so that this apparatus will not be discussedin further detail. A typical example of an ultrasonic inspectionapparatus that may be utilized for performing the method is theinstrument USM 25 of the applicant.

FIG. 1 also shows a second position of the probe 28 that is labelled at29. During inspection, the probe 28 is moved along the arrows 44. Themovement must be large enough to pass over the crack 26. At the anglebeam probe, which is shown in a dashed line, a radiation beam 46 isoutlined in addition to the main beam 30. The reader is referred to theDE-Book mentioned herein above for a definition of a radiation beam.From the position shown in a dashed line, the probe is displaced duringinspection sufficiently far to pass over the crack 26 and again reach aposition in which it is located outside of the crack 26, meaning inwhich it is located at the same distance as the dashed position of theangle beam probe 28, but on the other side. It should be noted here thatthe dashed position is very far away and that it is possible that thecrack 26 be again detected in the dashed position, but now after themain beam 30, which is shown in a dash-dot line, has been reflected atfirst from the back face 24, then from the front face 22 and from theretoward the crack 26.

In practice, during inspection, the emission angles alpha are about 450and typically range from 45-60°. This however does not mean that otherangles alpha are excluded. The probe frequencies are in the MHz range,for example 1-5 MHz. The ultrasonic pulses are emitted at a repetitionfrequency of 50-100 Hz, with much higher frequencies being possible; thesame applies for lower frequencies.

An inspection method is run as follows:

As can be seen from FIG. 1, the probe 28 emits ultrasonic pulses alongthe main beam 30. These ultrasonic pulses impinge either directly thecrack 26 or the back face 24. In both cases, they are reflected towardthe back face 24 or toward the crack 26 and are caused to return intothe probe 28 after angular reflection. In the position as shown in FIG.1, the main beam 30 first travels toward the back face 24, from there,after reflection, it travels along a short path to the crack 26 and fromthere back into the probe.

Each position of the probe 28 leads to an echo signal at a certaintravel time. Each new position has another travel time and another valuefor the echo signals. The echo signals received by the receiver module34 are amplified there and then digitalized in the A-D converter 36. Inthe memory 40, only the maximum amplitudes for one position of the probeand, as a result thereof, for one travel time are stored. Concurrently,the monitor displays the A-scan of the pulse that has just been emittedand/or the maximum values of all of the measurements performed duringmovement along the arrows 44. The maximum amplitudes for all theoccurring travel times form an envelope curve 48 such as illustrated inFIG. 3.

FIG. 2 shows the relative position between probe 28 and crack 26 inwhich the main beam 30 strikes the tip of the crack 26, whereas in FIG.1 the probe 28 is in a position in which it is quite near the root ofthe crack 26. In the position as shown in FIG. 2, about half of theradiation beam 46 is led past the crack 26 provided that the crack has acorresponding geometry, with about half the radiation beam beingreflected into the probe 28. This leads to an amplitude of the echosignal that is about 50% less than the maximum amplitude. The maximumamplitude is for example typically achieved in the position of the probe28 as shown in FIG. 1. It can be seen that the position in FIG. 2 is aposition permitting to detect the half-width t2-t1 of the flaw.

FIG. 2 also illustrates in a dashed line a probe 29 offset by 180°together with its main beam 30. This is to make it obvious that thecrack 26 can also be detected from the other side. The two measurementsas outlined in FIG. 2 can be combined; the mean value of the flaw depthobtained for the two directions of emission can be outputted as the flawdepth T.

FIG. 2 also illustrates an intentionally introduced test flaw 50; it isa serration of known depth and width. It serves to set the ultrasonicinstrument. This setting is performed as follows:

The flaw depth T50 of the test flaw is known. The test flaw 50 is nowinspected using the inspection method; an envelope curve is established,which is similar to the envelope curve for the flaw 26 of FIG. 3. In theenvelope curve 48 shown therein, a half-width is plotted at 50% of themaximum amplitude. It commences at the overall travel time t1 and endsat the overall travel time t2. This is performed in the same manner forthe envelope curve of the test flaw 50. Now, a proportionality factor kis computed from the quotient of the depth of the test flaw divided bythe half-width of said test flaw. This permits to now determine the flawdepth T of the crack 26 using the rule of three or the proportionalityfactor k according to the formulaT=k×half-width of the crack.

Put another way, the half-width obtained from the envelope curve shownin FIG. 3 is multiplied by the proportionality factor k; the result isthe crack depth T. The crack depth T is directly displayed on themonitor 38; in FIG. 3, the value 5.2 mm is indicated by way of example.

In FIG. 3, the maximum amplitude of the envelope curve 48 is 80%;accordingly, the half-amplitude is 40%. At other maximum amplitudes, thecomputer module 42 computes the associated half-amplitude. Moreover, theamplification can be varied according to the formula indicated hereinabove.

FIG. 3 further illustrates an actual echo 52 as it usually appears onthe A-scan. It does not entirely attain the height of an amplitude valuemeasured at an earlier stage for the same travel time t3, so that itwould not be taken into consideration for being stored in the memory 40.

The half-width t2 minus t1 is automatically calculated in the computermodule 42. For this purpose, current computations, which are known inthe art, are to be performed; they need not be discussed herein.

The envelope curve is also referred to as echo dynamic curve. Theproduct of the half-width and of the maximum amplitude is multiplied bythe proportionality factor k; the result is the crack depth.

The method has the advantage that an inspection report on the ultrasonicinspection performed may additionally include the stored envelope curvesand so on. Improved documentation is thus made possible. It is alsopossible to evaluate the envelope curves at a later stage from otherviewpoints.

Finally, FIG. 3 also shows two evaluation thresholds 54 and 56 at thebase of the envelope curve 48. They are located so as to lie just abovethe zero line. They are intersected by the envelope curve so that theenvelope curve is both above and beneath the thresholds. This is to makecertain that the envelope curve is completely registered, that is, thatthe probe has been moved sufficiently away from the crack 26. This isadvantageous for automatically performed measurements. It is however tobe understood that it suffices in principle to register the envelopecurve 58 until just beneath the half-value, this being sufficient forthe measurement in accordance with the invention.

1. A method for determining the size of a crack in a workpiece, morespecifically the depth of a crack in said workpiece, using theultrasonic pulse-echo method, said method involving the following methodsteps: a workpiece is chosen having a front face and a back face,wherein the workpiece exhibits a crack starting at the back face, anangle beam probe is placed on the front face, the angle beam probe sendsultrasonic pulses at an angle alpha into the workpiece and receives echosignals of said pulses, the angle beam probe is moved at least once overthe crack so that the radiation beam of the angle beam probe sweepsacross the entire crack, the received echo signals are digitalized andstored in a memory as pairs of echo signal values over travel time,whereby the stored pairs of values form a multitude and an envelopecurve is constructed of this multitude, wherein for the construction ofthe envelope curve the high values of the stored pairs are used, thesize of the crack is calculated from the width of the envelope curve ata predetermined partial amplitude and from the maximum amplitude of theenvelope curve.
 2. The method as set forth in claim 1, wherein severalecho amplitudes are obtained for an individual value of the travel time,and wherein only the echo amplitude having the highest value is stored.3. The method as set forth in claim 1, wherein the size of the crack isproportional to the product of the maximum amplitude of the envelopecurve and the width of the envelope curve at 50% of the maximumamplitude.
 4. The method as set forth in claim 1, wherein the angle beamprobe is a component part of an ultrasonic inspection apparatus, whereinsaid ultrasonic inspection apparatus further comprises a computer moduleand wherein said computer module outputs an output value representing aflaw size.
 5. The method as set forth in claim 1, wherein the angle beamprobe is moved several times across the crack.
 6. A device for carryingout the method as set forth in claim 1 for determining crack in aworkpiece using the ultrasonic pulse-echo technique, said devicecomprising: an angle beam probe being a component part of an ultrasonicinspection apparatus which ultrasonic inspection apparatus is furthercomprised of the following parts: a) a transmitter module and a receivermodule, b) an A/D (analog-digital) converter that is connecteddownstream of the receiver module, c) a memory for storing values echosignals which values echo signals are in the form of pairs and arereceived from the transmitter module and are digitalized by the A/Dconverter together with the respective travel time, only the highestecho amplitude obtained being stored for every individual travel timeand d) a computer module for computing the depth of the crack out of themaximum amplitude stored and from a width dimension of the envelopecurve as stored.
 7. The device as set forth in claim 6, wherein theultrasonic apparatus comprises a monitor for displaying the envelopecurve.
 8. The method as set forth in claim 1, wherein the angle beamprobe is moved several times back and forth over the crack.
 9. A methodfor determining the depth of a crack in a workpiece, using the so-calledultrasonic pulse-echo method, said method involving the following steps:a workpiece is chosen having a front face and a back face, wherein theworkpiece exhibits a crack starting at the back face, an angle beamprobe is placed on the front face, the angle beam probe sends ultrasonicpulses at an angle alpha into the workpiece and receives echo signals ofsaid pulses, the angle beam probe is moved at least once over the crackso that the radiation beam of the angle beam probe sweeps across theentire crack, the received echo signals are digitalized and stored in amemory as pairs of echo signal values over travel time, whereby thestored pairs of values form a multitude and an envelope curve isconstructed, wherein for the construction of the envelope curve the highvalues of the stored pairs are used, the size of the crack is calculatedfrom the width of the envelope curve at a predetermined partialamplitude and from the maximum amplitude of the envelope curve.